1. Field of Invention
The present invention relates to the field of sensors, particularly sensors that indicate local changes in conditions on articles, and more particularly in the field of positionable sensors that can be applied to a surface, used in a sensing procedure, and then removed from the surface. The invention also relates to flexible electrical sensors for use in medical applications to provide information or measurement on the stress, elongation, pressure, or load that is applied to or placed upon the sensor. The present invention may be utilized as part of a medical device system to provide information or measurement of stress, elongation, pressure, or load in the insertion process or the performance of the medical device system.
2. Background of the Art
Piezoresistivity, the change in electrical resistivity under stress or strain, may be a property of electrical conductive materials, as is with electrically conductive rubbers containing conducting nanotubes such as single-wall or multi-wall carbon nanotubes. The piezoresistive effect describe the changing resistivity of a semiconductor due to applied mechanical stress. The piezoresistive effect differs from the piezoelectric effect. In contrast to the piezoelectric effect, the piezoresistive effect only causes a change in electrical resistance; it does not produce an electric potential. Piezoresistivity is defined by
      ρ    σ    =            (                        ∂          ρ                ρ            )        ɛ  
Where                ∂ρ=Change in resistivity        ρ=Original resistivity        ε=Strains        
When a voltage is applied to such a material, and stress or strain is applied to the material, the electrical resistance of the material changes in response to the stress, and the resulting change in resistance can be measured in the change in current through the material. Applying a constant voltage to a conductive nanotube flexible polymer composite and monitoring the output current of the conductive composite while a stress or strain is applied, gives a direct mathematical correlation between the change in resistivity caused by the forces applied and the change in local dimensions of the article formed by the material. Flexibility of nanotube rubber composite, as well as the large change in resistivity associated with an electrically conductive nanotube composite, has an advantage over piezoelectric sensors, especially over large numbers of deformations. The electrically conductive nanotube rubber composite also has advantages over electrically capacitive sensors due to the relatively large change in resistivity is well above the back ground electrical noise level, thus potentially better suited for environments where the electrical signal to noise ratio may be a challenging problem for capacitive sensors.
U.S. Patent Application Publication No. 20110319755 (Stein) describes a sensing insert device for measuring a parameter of the muscular-skeletal system. The sensing insert device can be temporary or permanent. The sensing module is a self-contained encapsulated measurement device having at least one contacting surface that couples to the muscular-skeletal system. The sensing module comprises one or more sensing assemblages, electronic circuitry, an antenna, and communication circuitry. The sensing assemblages are between a top plate and a bottom plate in a sensing platform. The bottom plate is supported by a ledge on an interior surface of a sidewall of a housing. A cap couples to top plate. The cap is adhesively coupled to the housing. The adhesive is flexible allowing movement of the cap when a force, pressure, or load is applied thereto.
U.S. Patent Application Publication No. 20110316522 (Shinobu) provides a sensing device that holds a piezoelectric sensor and a channel forming member placed on the sensor in a closely contacted state while maintaining a shape of space of a passage space formed inside the device. In a sensing device that senses a substance to be sensed based on a variation in an oscillation frequency caused by an absorption of the substance to be sensed in an absorption layer provided on a piezoelectric resonator of a piezoelectric sensor, a holding member holds the piezoelectric sensor and a channel forming member that forms a passage space through which a sample fluid passes on an upper surface side of the sensor, in a vertically stacked state. A cover member is placed on the channel forming member, and a pressing part which is raised/lowered by a first raising/lowering mechanism presses the cover member 1510 placed on the channel forming member downward with a previously set force.
U.S. Patent Application Publication No. 20110306824 (Perron) describes an implantable system comprises a housing that includes a flexible reservoir and a piezoelectric sensor system. The flexible reservoir is coupled to an inflatable portion of a gastric band via a fluid inlet/outlet. The flexible reservoir contains a fluid and has an expanded configuration and a contracted configuration. An access port may be coupled to the flexible reservoir and/or the gastric band to facilitate filling and draining the reservoir and/or the gastric band. A movable wall is slidably positioned around the flexible reservoir to move the flexible reservoir between the expanded configuration and the contracted configuration to move the fluid into and out of the inflatable portion of the gastric band. A driving mechanism is positioned around the movable wall and is capable of changing the size of the movable wall to compress or expand the flexible reservoir. A motor, coupled to the driving mechanism, may actuate the driving mechanism.
U.S. Patent Application Publication No. 20090293631 (Radivojevic) describes a sensing device for measuring flexural deformations of a surface. Such a sensing device may be used as a user interface in portable electronic devices. The sensing device comprises at least one cell. The cell comprises a first electrode, a central electrode, a second electrode, a first piezoelectric sensing layer placed between the first electrode and the central electrode, a second piezoelectric sensing layer placed between the central electrode and the second electrode, and a circuit connected to the first, second and the central electrodes. The circuit is configured to measure a first electrical signal between the first electrode and the central electrode, and a second electrical signal between the second electrode and the central electrode. At least one of the first electrical signal and the second electrical signal is responsive to an external stress applied on the sensing device.
U.S. Patent Application Publication No. 20090308742 (Paranjape) relates to a system and method that co-locates in a small flexible, configurable system and multi-level substrate sampling, rapid analysis, bio-sample storage and delivery functions to be performed on living tissues or matter obtained from living organisms. The types of the sampling may include chemical, biochemical, biological, thermal, mechanical, electrical, magnetic and optical, sampling. In general, the analysis performed at the point of sampling measures the sample taken and records its value. The bio-sample storage function encapsulates a small sample of analyte and preserves it for subsequent examination or analysis, either on the organism by the system or at a remote location by an independent analysis system. Once stored, the sample can provide a record of a biological state at the precise time of sampling. The delivery at the point of sampling can include chemical, biochemical, thermal, mechanical, electrical, magnetic and optical stimuli.
U.S. Pat. No. 7,730,547 (Barrera) is directed toward devices comprising carbon nanotubes that are capable of detecting displacement, impact, stress, and/or strain in materials, methods of making such devices, methods for sensing/detecting/monitoring displacement, impact, stress, and/or strain via carbon nanotubes, and various applications for such methods and devices. The devices and methods of the present invention all rely on mechanically-induced electronic perturbations within the carbon nanotubes to detect and quantify such stress/strain. Such detection and quantification can rely on techniques which include, but are not limited to, electrical conductivity/conductance and/or resistivity/resistance detection/measurements, thermal conductivity detection/measurements, electroluminescence detection/measurements, photoluminescence detection/measurements, and combinations thereof. All such techniques rely on an understanding of how such properties change in response to mechanical stress and/or strain.
U.S. Patent Application Publication No. 20090308742 20110275502 (Eichhorn) discloses an electrically conductive roller, belt or mat for an elastomeric printing surface. The elastomeric material may be comprised of a selection of base rubber materials chosen from silicone, ethylenic elastomers and rubbers such as ethylene propylene diene monomer based elastomers, or class-M rubbers (EPDM), FKM (fluorocarbon or perfluorcarbon polymers.elastomers and rubbers, especially fluorelastomers as defined in ASTM D1418), polyurethanes and other elastomeric rubber polymers to which nanotubes are added to form a nanotube rubber composite. Specifically, the electrically conductive roller, belt or mat elastomeric material composition is comprised of a carbon nanotube silicone rubber utilizing a platinum cured liquid silicone rubber with very small loadings of carbon nanotubes. The patent application describes the carbon nanotube composite material physical & electrical properties as applied to the printing component and various embodiments including carbon nanotube composites bonded to other materials such as metals and thermoplastics.
The M class includes rubbers having a saturated chain of the polymethylene type. Dienes currently used in the manufacture of EPDM rubbers are dicyclopentadiene (DCPD), ethylidene norbornene (ENB), and vinyl norbornene (VNB). The ethylene content is around 45% to 75%. The higher the ethylene content the higher the loading possibilities of the polymer, better mixing and extrusion. Peroxide curing these polymers give a higher crosslink density compared with their amorphous counterpart. The amorphous polymers are also excellent in processing. This is very much influenced by their molecular structure. The dienes, typically comprising from 2.5% up to 12% by weight of the composition, serve as crosslonks when curing with sulphur and resin, with peroxide cures the diene (or third monomer) functions as a coagent, which provide resistance to unwanted tackiness, creep or flow during end use.
Significant background technology on piezeresistance in sensors is provided at www.mech.northwestern.edu/FOM/LiuCh06v3—072505.pdf (which is submitted with this application).
“A carbon nanotube/polymer strain sensor with linear and anti-symmetric piezoresistivity,” Gang Yin et al. Published online before print Apr. 26, 2011, doi: 10.1177/0021998310393296 Journal of Composite Materials June 2011 vol. 45 no. 12 1315-1323 describes improved piezoresistive sensors in the aerospace industry. Additional Journal disclosures in this area of technology include “Flexible Strain Sensor Based on Carbon Nanotube Rubber Composites,” Jin-Ho Kim et al., Nanosensors, Biosensors and Info-Tech Sensors and Systems 2010, edited by Vijay K. Varadan, Proc. Of SPIE Vol. 7646, 7646ON; “Piezoresistive response of epoxy composites with carbon nanoparticles under tensile load,” Wichmann, Malte H. G., et al., PHYSICAL REVIEW b80, 245437 (2009, THE American Physical Society; “Supersensitive linear piezoresistive property in carbon nanotubes/silicone rubber nanocomposites,” Zhi-Min Dang et al., Journal of Applied Physics, 104, 024114 (2008), American Institute of Physics.
All of the references cited herein are incorporated by reference in their entirety. It is desirable to find additional utility for sensors in the manufacturing and medical fields.